The cardiac cycle can be described as the activation of certain specialized heart conduction cells in a predictable sequence which leads to a coordinated and sequential contraction of the atrial and ventricular muscle fibers. This sequence of events, culminating in a cardiac contraction, leads to effective circulation of blood to vital parts of the body. Normally, heart activation is an ordered sequence of electrical depolarization and repolarization from the sino-atrial node to the ventricular fibers. Effective cardiac contraction is thus dependent upon the anatomical distribution and electrical properties of these specialized fibers. Voltage variations are generated from the depolarization and repolarization of the specialized cardiac fibers which creates electrical fields that reach the body surface. A surface electrocardiogram is a graph of these voltage variations plotted over time.
Cardiac monitoring is a critical component of all emergency rooms, critical care units, and telemetry beds. Abnormalities of cardiac rhythm may be the first sign of an impending cardiopulmonary arrest and can be prevented with early detection and treatment. The monitors in current use utilize surface electrodes located on the body connected by wires to an electrocardiographic machine which allows the detected heart signal to be displayed on a paper strip or a monitor screen. Alternatively, these wires can be connected to a small but cumbersome relay box hung around the patient's neck. Often these relay boxes or monitors cannot be used during critical tests such as with imaging studies using magnetic resonance.
The normal electrocardiogram is obtained from one of three types of electrical connections. These connections are known as limb leads, augmented limb leads, and precordial leads. Limb leads are bipolar leads in which two electrodes are used to detect electrical variations at two points and the difference between these signals is what is displayed. Augmented limb leads are unipolar in that one electrode is used to detect electrical variations in potential at one site on the body and then compared to an electrode located at a site where electrical activity does not vary much with the cardiac cycle. For instance, one electrode could be connected to either a right arm, left arm, or leg and the generated signal can be compared to a signal from an electrode located at a point which does not vary significantly in electrical activity during cardiac contraction. The precordial leads are also unipolar, but unlike the augmented limb leads, one or more precordial electrodes are connected to the chest wall. The generated signal(s) is compared to a signal generated by an electrode located at a point which again does not vary significantly with the cardiac cycle. The current state of the art requires wire connections between these surface electrodes to generate the resultant electrocardiogram.
In an ideal lead system for monitoring electrical activity of the heart, the electrodes should be perpendicular to each other, the amplitude detected by each electrode should be roughly equal, and each electrode should have the same signal strength and direction for all points in the heart where electrical forces are generated. These ideal electrodes have been termed corrected "orthogonal leads." Orthogonal lead systems have recently been constructed on implantable subcutaneous cardiac monitors for detecting and recording episodes of cardiac syncope. A disadvantage of these implantable monitors is the need for surgical placement and the limited usefulness for short term cardiac monitoring situations which occur in emergency or operating rooms and in the critical care facility or elsewhere.